Generated analog electrical signals are of course commonly converted and utilized in digital form. In a variety of such schemes the signal-to-noise ratio (S/N) of the signal-generating device is much better than is the dynamic range of the associated analog-to-digital (A/D) convertor. This is true, for example, in Fourier transform infrared (FT-IR) spectrometers, in which a mercury cadmium telluride (MCT) photoconductive detector is typically used for detecting radiation in the mid-infrared region of the spectrum. Most such FT-IR instruments have 16-bit A/D convertors for digitizing the amplified MCT signal; because of the relatively high S/N of the detector, however, digitization of its signals by a 20-bit convertor is feasible.
An FT-IR spectrometer will typically generate an A.C. signal that, for the most part, oscillates about zero but that has a small region (the "center burst") near the zero path difference (ZPD) at which the signal peaks to 100 to 1000 times that level. Consequently, it is conventional to enhance the dynamic range of the FT-IR signal by gain ranging; that is, to set the amplification gain to some known, relatively high level during data acquisition, and to decrease it to some other known level in the region of the ZPD so as to avoid overloading of the A/D convertor. The technique is however disadvantageous for a number of reasons. Not only must the two gain levels be known very accurately, but there is also a tendency for switching noise generation because the data-acquisition rate can often exceed tens of kilohertz, thus subjecting the switch to fast settling time demands; it is very difficult, moreover, to remove the slight offsets from zero that can occur during the amplification changes.
The prior art teaches various means for effecting signal compression. Nonlinear devices, such diodes, transistors and FET's, have for example been incorporated into the feedback loops of Op Amps, as in U.S. Pat. No. 4,899,115, issued to Christian on Feb. 6, 1990, and U.S. Pat. No. 4,627,094, issued to Scholz on Dec. 2, 1986. U.S. Pat. No. 4,349,787, issued to Laupman on Sep. 14, 1982, and U.S. Pat. No. 4,400,583, issued to Bloy on Aug. 23, 1983, provide apparatus in which devices incorporated into amplifier feedback loops serve as adjustable gain controls, which are in turn controlled by other, secondary sensing circuitry.
These prior art compression algorithms also suffer however from certain deficiencies and drawbacks, rendering them unsuitable or unusable for many applications, including that to which the instant invention is (in its preferred embodiments) directed. This is attributable primarily to the fact that the compression functions of the devices involved are either insufficiently known, are susceptible to temperature changes, or are time averaged, thus making computation of the digitized signal, and emulation of the original uncompressed signal, inconsistent and difficult, if indeed possible at all.
Accordingly, it is the broad object of the present invention to provide a novel method, apparatus and system for enhancing analog electrical signal properties, by compression.
More specific objects are to provide such a method, apparatus and system by which the signal-to-noise ratio of the signal can be maximized so as to make use of the entire dynamic range of a digitizer, and to provide a method and system for digitally decompressing the signal so as to faithfully emulate the original, uncompressed signal.
Additional objects of the invention are to provide such a method, apparatus and system in which compression is effected in a manner that is highly efficient and effective, and by means that is relatively incomplex, highly reliable, stable, convenient to calibrate and utilize, and relatively inexpensive and facile to produce.